If they're measuring the strength of a person's breath, for instance. Even though a lot of people might use "consistente" for "consistent," in RAE "consistente" doesn't have this definition. When the person blows into the machine (if that's what the context is), they're looking to see if it falters, weakens, doesn't remain the same.

consistir (Del lat. consistĕre).

1. intr. Dicho de una cosa: Estribar, estar fundada en otra. Su trabajo consiste en corregir pruebas.
2. intr. Ser efecto de una causa.
3. intr. desus. Dicho de una cosa: Estar incluida o encerrada en otra.
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constante: 3. adj. Dicho de una cosa: Persistente, durable.